With the development of research on, and commercial use of, the elastomeric copolymers of vinylidene fluoride with fluorinated or chlorofluorinated olefins, or with fluorinated vinyl ethers, certain well-defined requirements have become increasingly critical. These include:
a general improvement of the rheological characteristics and processability (workability) of the mixes based on said elastomeric copolymers;
the possibility of obtaining, from the mixes, relatively soft manufactured articles, i.e., articles having a hardness lower than 55 IRHO; and
easy extrusion of the mixes to obtain manufactured articles having a complicated profile and, also, of considerable size.
It has been attempted to satisfy the latter-day requirements by resort to processing aids of the plasticizing type, or to various lubricants.
The processing assistants heretofore used for the purpose, such as polyesters, polyethylene, pentaerythritol tetrastearate, wax, and like agents did not prove suitable because they cause appreciable deterioration of the typical characteristics of the fluorinated elastomers.
In fact, in order to impart to the elastomers in question the well known properties of heat-resistance and resistance to fluids, it is indispensable to carry out a "heat stabilization" (post-vulcanization) at temperatures comprised between 200.degree. C. and 250.degree. C., over a period of about 24 hours.
During this operation, the standard plasticizing agents (e.g., glycols, stearates, organic phosphates, oleates, phtalates, etc.), volatilize and/or decompose. This causes a considerable deterioration of the mechanical properties of the manufactured end products. In particular, it has the following effects on the end products:
an excessive increase in hardness;
a decrease in the resistance to the permanent set under compression ("compression set");
a decrease in the elasticity characteristics (low value of the elongation and high value of the elasticity coefficient);
poor thermal resistance which hinders its use under severe application conditions, i.e., at temperatures higher than 180.degree. C.
Summing up, it may be said that the benefits achieveable in the processing stage lead to unacceptable disadvantages in the finished products.
Thus, the use of plasticizers and lubricants is limited to small quantities (1-3 p. by w./100 p. rubber) that do not appreciably alter the final properties of the vulcanized products, but which, on the other hand, are insufficient to satisfy the requirements mentioned above.
In particular, it has been noticed that the use of perfluorosilicon oils results in reduction of the hardness of a product by 4-5 points, that is, down to the lowest limit of 50 IRDH, although it presents difficulties in admixing and incorporating said oils into the fluoro-elastomers.
Moreover, there occurs a drop in vulcanization rate and a worsening of the compression set value and the resistance to heat.
The use of very low molecular weight fluorinated polymers, having the consistency of waxes, such as: "Viton LM", produced by Du Pont, was adopted in order to impart to the molecules better rheological characteristics, particularly during extrusion. It is thus possible to obtain extruded articles of great dimensions and with a complicated profile. However, the use of such a "plasticizer", in a ratio of 10 to 20 parts by weight for 100 parts of fluoro-elastomer, has the following disadvantages:
it is possible only with polyamine-based conventional vulcanizing systems, while it is not compatible with more recent vulcanizing systems based on bisphenols and accelerators incorporated in the fluoro-elastomers; and
it causes slowing down of the vulcanization, which necessitates increase of the dwell time in the press.